The registration period is different depending on your nationality, the study cycle for which you are applying, your status (assimilated student or not). You will find the deadlines that concern you on this page of the Registration office. You will then have to submit an online application following the procedure. Within a maximum of 10 weeks, you will be notified by e-mail of the result reserved for your application. Caution for the studies limited-entry study programmes, the procedure of enrolment is peculiar (in French).

Here is a tutoriel video which explains the steps to enrol.